1. Field of the Invention
The present invention relates to a standardized mechanical interface (SMIF) container for transferring workpieces such as reticles in a semiconductor or reticle fab, and in particular to a container including a support structure mounted for supporting the workpiece in a secure position without contacting the upper or lower surfaces of the workpiece.
2. Description of the Related Art
Semiconductor devices are made up of as many as fifty individual patterned layers of silicon, silicon compounds and metals. During fabrication of these devices, the pattern for each of these layers is contained on a mask called a reticle. A reticle is an optically clear quartz substrate on which a pattern has been formed by photolithography or other such processes. In particular, a layer of photoresist is applied on a chrome coated reticle blank. Thereafter, the pattern for a particular layer to be formed on a semiconductor wafer is transferred onto the reticle as for example by a laser pattern generator or e-beam. After pattern generation on the photoresist, the exposed portions of the photoresist are removed to leave the unwanted portions of the chrome layer exposed. These unwanted portions are then etched away. The remaining photoresist is then removed in a process which leaves the clean pattern on the surface on the reticle.
In order to keep the surface of the reticle clean, a thin transparent sheet called a pellicle is mounted a short distance away from the surface of the reticle containing the pattern. This ensures that any microscopic dust that settles on the reticle will be out of focus during the exposure process so as not to affect the pattern formed on the silicon wafer.
During fabrication of the reticle, it is important to minimize airborne particle fluxes onto the surface of the reticle on which the pattern is being formed, as any such particles can corrupt the pattern. Even after formation of the pattern and affixation of the pellicle, larger, or macro, contaminants can settle on the reticle which can interfere with pattern transference onto the semiconductor wafer. It would therefore be advantageous to shield the reticles from the external environment during reticle fabrication, during transfer of the reticle from the reticle fab to the semiconductor fab, and during usage of the reticle in the semiconductor fab.
In addition to exposing the reticle to airborne particulates, physical handling of a reticle during transfer can also damage a reticle. Common causes of damage when handling reticles include scratches, electrostatic discharge onto the reticle, and cracking of the reticle and/or pellicle.
In a semiconductor wafer fab, it is known to store and transfer workpieces such as semiconductor wafers using a standard mechanical interface, or SMIF, system. The SMIF system was developed by the Hewlett-Packard Company and disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto workpieces such as reticles and semiconductor wafers during storage and transport of the workpieces through the fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the workpieces is essentially stationary relative to the workpieces and by ensuring that particles from the ambient environment do not enter the immediate workpiece environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
A SMIF system has three main components: (1) sealed containers, having a minimal volume, used for storing and transporting workpieces and/or cassettes which hold the workpieces; (2) enclosures placed over access ports and workpiece processing areas of processing equipment so that the environments inside the containers and enclosures (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload workpieces and/or workpiece cassettes from a sealed container without contamination of the workpieces from external environments.
SMIF systems have not conventionally been used in reticle fabs. The reticles are manually transferred using hand-held grippers through the various fabrication processes. Thus, the reticle is subject to the above-discussed disadvantages, namely, airborne particulates settling on the reticle, scratches, electrostatic discharge onto reticles and cracking of the reticle and/or pellicle.
In semiconductor wafer fabs, reticles are conventionally transferred in SMIF containers, or pods, configured to transfer individual or multiple reticles. The containers are in general comprised of a door which mates with a shell to provide a sealed environment in which the reticles may be stored and transferred. In order to transfer reticles between a SMIF container and a process tool within a fab, a pod is typically loaded either manually or automatedly on a load port on a front of the process tool. Once the pod is positioned on the load port, mechanisms within the port door unlatch the door from the shell so that the reticle may be transferred from within the container into the process tool.
Conventional SMIF containers include horizontal shelves along the side of the container on which an underside of the reticle is supported. Such conventional reticle SMIF containers have several disadvantages. First, it is desirable to minimize contact with the upper and lower surfaces of the reticle. Any such contact may generate particles and/or affect the pattern etched in the reticle. Additionally, it is desirable to avoid contact with vertical edges of the reticle.
A second disadvantage to conventional reticle containers is that the reticle is not securely supported in a fixed position within the container. In order to minimize movement of the reticle within the container, conventional reticle containers include vertical restraints at the front and back of the container which contact the front and back edges of the reticle to secure it in place. The front retainer moves into and out of engagement with the reticle edge when the pod shell and door are coupled and decoupled, respectively. The back vertical restraint is sloped outward away from the reticle from its bottom to top. This allows the reticle to be lifted up off of the shelves without scraping against the back restraint as it moves upward. In addition to contacting the edges of the reticle, a further disadvantage to this restraint system results from the fact that the slope of the back wall makes it difficult to provide the desired clamping force on the reticle by the front restraint upon closing the container, because such forces can cause the reticle to ride up the slope of the back restraint. Additionally, it is fairly easy for the reticle to become dislodged from between the restraints upon a shock or jolt to the container.
A still further disadvantage to conventional reticle containers is that they do not precisely control the position of a reticle in its transport container. Variation in the expected reticle position in the container may result in mishandling of the reticle or damage to the reticle as a result of unexpected contact with the reticle by a mechanism used to transport the reticle away from the container.